There are various light emitting components and devices based on a light emitting element, wherein the color or wavelength of the light emitted by the light emitting element must be converted into one or more other colors or wavelengths. A typical example is a white LED, where the initial wavelength range e.g. in the ultraviolet, blue, or green portion of the spectrum is converted into white light comprising several wavelength ranges. White light is used e.g. in illumination applications. Other typical examples of luminescent components and devices where wavelength conversion is used are various displays for presenting alphanumerical and graphical information, backlight units, as well as fluorescent lamps.
The wavelength conversion is typically made by means of a luminescent material receiving and absorbing the light of a first wavelength range emitted by the light emitting element, and emitting the absorbed energy at one or more other wavelength ranges.
There is a variety of different luminescent materials, often called “phosphors”, for this purpose known in the art. However, the longest emitting afterglow materials contain either rare earth elements (lanthanides) or other heavy metal elements. Examples of such long and brightly emitting commercially used materials are e.g. Sr2MgSi2O7:Eu,Dy (blue color), CaAl2O4,Eu,Nd (blue color), Sr4Al14O25:Eu,Dy (blue/green color), SrAl2O4:Eu,Dy (green color) and Y2O2S:Eu,Mg,Ti (red color). The longest persistent reported is in the near infrared (NIR) and it has been observed for Zn3Ga2Ge2O10:Cr (invisible emission, over 360 h persistence). The prior art also recognizes some white afterglow materials of which e.g. CdSiO3:Dy, Y2O2S:Ti, CaSnO3:Tb, and Ca2MgSi2O7:Dy have the longest afterglows.
All of the best performing materials thus contain rare earth metals and/or other heavy metal elements. The necessity to use rare earth metals as well as elements like Bi and Ge increase the production costs of these materials. Moreover, heavy metal elements, such as Cd, are commonly not considered healthy either for the environment or humans. The inventors have therefore recognized the need for a new material providing a long afterglow without any rare earth metals or other heavy metal ions.